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Modify outgoing messages in Outlook with a VB add-in

Hello all,
Here is my situation. I have been tasked at my job to create an add-in for Microsoft Outlook 2007 that will display a dialogue box when a user sends out an e-mail that will ask them to choose a classification of that e-mail.
I would like that dialogue to be a windows form, which I know how to code with VB.net. I, however, do not know how to write add-ins to interface with Outlook to accomplish this. I have figured out how to load the templates that come with VS 2008 and there is one for Outlook 2007, but I can not figure out how to intercept e-mails or how to display windows forms.

Please any help would be very much appreciated.
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